1. Peel and slice the potatoes.

2. Peel and slice the onions.

3. Cut the chicken breast tenders into small pieces.

4. Grease a baking dish with butter.

5. Layer a portion of potato slices in the baking dish, then distribute a layer of onion rings and chicken breast pieces on top.

6. Season with salt and pepper.

7. Add another layer of potato slices, onion rings, and chicken breast pieces, and season with salt and pepper again.

8. Grate the Gouda cheese and sprinkle it over the potato-onion-chicken mixture.

9. Heat the milk until it is almost boiling, then pour it over the gratin.

10. Bake the gratin in a preheated oven at 180 degrees Celsius (350 degrees Fahrenheit) for about 40-45 minutes, until it is golden brown and crispy.

11. Serve hot and enjoy!